var code = "";
/**绘制验证码图片**/
function drawPic() {
	code = "";
	var canvas = document.getElementById("vcode_img");
	var width = canvas.width;
	var height = canvas.height;
	//获取该canvas的2D绘图环境 
	var ctx = canvas.getContext('2d');
	ctx.textBaseline = 'bottom';
	/**绘制背景色**/
	ctx.fillStyle = randomColor(180, 240);
	//颜色若太深可能导致看不清
	ctx.fillRect(0, 0, width, height);
	/**绘制文字**/
	var str = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890';　　　　　 //生成四个验证码
	for(var i = 1; i <= 4; i++) {
		var txt = str[randomNum(0, str.length)];
		code = code + txt;
		ctx.fillStyle = randomColor(50, 160);
		//随机生成字体颜色
		ctx.font = randomNum(15, 40) + 'px SimHei';
		//随机生成字体大小
		var x = 10 + i * 25;
		var y = randomNum(25, 35);
		var deg = randomNum(-45, 45);
		//修改坐标原点和旋转角度
		ctx.translate(x, y);
		ctx.rotate(deg * Math.PI / 180);
		ctx.fillText(txt, 0, 0);
		//恢复坐标原点和旋转角度
		ctx.rotate(-deg * Math.PI / 180);
		ctx.translate(-x, -y);
	}


	/**绘制干扰线**/
	for(var i = 0; i < 3; i++) {
		ctx.strokeStyle = randomColor(40, 180);
		ctx.beginPath();
		ctx.moveTo(randomNum(0, width / 2), randomNum(0, height / 2));
		ctx.lineTo(randomNum(0, width / 2), randomNum(0, height));
		ctx.stroke();
	}
	/**绘制干扰点**/
	for(var i = 0; i < 50; i++) {
		ctx.fillStyle = randomColor(255);
		ctx.beginPath();
		ctx.arc(randomNum(0, width), randomNum(0, height), 1, 0, 2 * Math.PI);
		ctx.fill();
	}
	return code;
}


/**生成一个随机数**/
function randomNum(min, max) {
	return Math.floor(Math.random() * (max - min) + min);
}
/**生成一个随机色**/
function randomColor(min, max) {
	var r = randomNum(min, max);
	var g = randomNum(min, max);
	var b = randomNum(min, max);
	return "rgb(" + r + "," + g + "," + b + ")";
}